Preheat your oven to 375°F (190°C).
Place the flank steak on a cutting board. Using a sharp knife, carefully slice the steak horizontally without cutting all the way through, creating a pocket for the filling.
In a large bowl, combine the chopped spinach, ricotta cheese, mozzarella cheese, sun-dried tomatoes, garlic, oregano, salt, and black pepper. Mix until well combined.
Spread the spinach and cheese mixture evenly inside the steak pocket.
Roll the steak tightly from one end to the other, keeping the filling inside. Tie the roll securely with kitchen twine in several places.
Heat olive oil in a large oven-safe skillet over medium-high heat. Sear the Stuffed Flank Steak for 4–5 minutes on each side until browned.
Transfer the skillet to the oven and bake for 20–25 minutes, or until the steak reaches your preferred doneness (about 130°F for medium-rare).
Remove from the oven and allow the steak to rest for 10 minutes.
Cut and remove the twine, then slice into 1-inch rounds and serve warm.